Biomolecular arrays have quickly developed into an important tool in life science research. Microarrays, or densely-packed, ordered arrangements of miniature reaction sites on a suitable substrate, enable the rapid evaluation of complex biomolecular interactions. Because of their high-throughput characteristics and low-volume reagent and sample requirements, microarrays are now commonly used in gene expression studies, and they are finding their way into significant emerging areas such as proteomics and diagnostics.
The reaction sites of the array can be produced by transferring, to the substrate, droplets containing biological or biochemical material. A variety of techniques can be used, including contact spotting, non-contact spotting, and dispensing. With contact spotting, a fluid bearing pin leaves a drop on the surface when the pin is forced to contact the substrate. With non-contact spotting, a drop is pulled from its source when the drop touches the substrate. With dispensing, a drop is delivered to the substrate from a distance. Reaction sites on the array can also be produced by photolithographic techniques (such as those employed by Affymetrix or NimbleGen, for example).
The quality of the reaction sites directly affects the reliability of the resultant data. Ideally, each site would have a consistent and uniform morphology and would be non-interacting with adjacent sites, so that when a reaction occurred at a given site, a clear and detectable response would emanate from only that one site, and not from neighboring sites or from the substrate. To reduce the overall size of an array while maximizing the number of reaction sites and minimizing the required reagent and sample volumes, the sites on the array should have the highest possible areal density.
With the present microarray technology, which is dominated by the use of flat substrates (often glass microscope slides), areal density is limited. To increase the signal from a given reaction site, the interaction area between the fluid and the substrate should be maximized. One way to do this is by using a surface that promotes wetting. A flat surface that promotes wetting, however, can lead to spots (and thus sites) having irregular shapes and compositions. A flat wetting surface can also lead to the spreading of fluid from its intended site into neighboring sites. Thus, flat surfaces are intrinsically limited by fluid-surface interactions that force a tradeoff between the desired properties of the reaction sites.
To make the sites more uniform, the surface can be made non-wetting. Unfortunately, this reduces the interaction area between the fluid and the surface and therefore reduces the signal that would otherwise be obtainable. In addition, since droplets do not adhere well to a flat non-wetting surface, deposition volumes can vary from site to site, and droplets can slide away from their intended place.
There is still a need for an improved biomolecular microarray apparatus that has a high areal density of sites and that permits the collection of data with good signal/noise ratio. Such an apparatus would ideally have sites of consistent and uniform spot morphology.